The invention relates to a demodulator that is based on the principle of peak detection for demodulating amplitude modulated alternating signals. The invention also relates to a contactless communication device including such a demodulator.
Demodulators may be used in the field of long range radio frequency (RF) communications. Portable contactless communication devices, such as contactless smart cards, electronic tickets or swipe cards, operate based on communicating by a magnetic field with a read and/or write interrogating member, generally referred to as a reader. These contactless devices generally comprise a microcircuit connected to a parallel LC type resonant circuit. The induction coil L is an external antenna, while the capacitor C is integrated with the microcircuit. The assembly forms what is commonly termed a tuning circuit.
As an example, in certain contactless smart card applications, the reader emits a signal having a carrier frequency of 13.56 MHz. This emitted signal allows the contactless card to be supplied by induction with the energy necessary for it to operate, and a establishes communications between the card and the reader according to an established protocol. Thus, when the contactless card penetrates into the emission field of the reader, it communicates with the latter by a modulation operation which includes modifying at least one parameter of the carrier.
The contactless device receives from the reader a signal that is amplitude modulated by a tuning circuit. The interpretation of the message of thee reader by the contactless device is performed by a demodulation operation which includes extracting the modulated signal from the carrier. The frequency of the modulated signal is much lower than that of the carrier, generally tens of kHz.
The quality and reliability of the radio frequency RF communications are directly related, among other things, to the distance between the reader and the contactless device. The distance or the range of the RF communications between the reader and the contactless device depends on several parameters, such as the tuning frequency between the resonant circuit of the contactless device and the emission frequency of the reader signal, as well as the quality of the demodulation of the modulated signal.
The quality of the demodulation of the modulated signal depends directly on the distance between the contactless device and the reader, and also on the speed of displacement of the device in the emission field of the reader. The greater the range and the stealthier the device, the more the demodulation will be erroneous.
A block diagram of the input stages of the contactless device is illustrated in FIG. 1. A resonant circuit, centered on the carrier frequency, receives a modulated signal. A rectifier bridge generates a DC voltage in order to supply the contactless device. The voltage Vdb at the output of the rectifier bridge represents the DC voltage after rectification and contains both the energy necessary for the self-supply of the contactless device and the information of the modulated signal.
For applications using 100% amplitude modulation, a diode isolates the resonant circuit from the load and thus eliminates any possibility of current returning toward the resonant circuit. A limiter permits the supply voltage Vdd to be maintained below a threshold, such as 4 V, for example. A resistor is advantageously placed between Vdb and the diode in order to isolate the signal modulated on Vdb. Thus, the demodulation of the modulated signal coming from the reader occurs directly from the signal Vdb at the output of the rectifier bridge of the contactless device.
A conventional amplitude demodulation device is illustrated schematically in FIG. 2. The signal Vdb is first treated by an RC type low pass filter to eliminate components of the carrier wave, to extract from it the envelope of the modulated signal, commonly referred to as the reference modulating signal Vmod, and its DC component, DC. A cutoff frequency of several tens of kHz can be chosen for this first filter. The DC level is then extracted by a new low pass filter having a cutoff frequency lower than the frequency of the modulating signal Vmod, such as by several kHz, for example. The demodulation signal Vdemod can then be obtained by comparing the reference modulating signal Vmod and its DC level.
This type of demodulation device has limitations due to its own structure. The DC level is widely variable as a function of the positioning of the card in the field of the reader, and of its speed of displacement. This makes the generation of a reliable comparison level difficult to permanently obtain. This problem is even accentuated for applications where a great range, such as 50 cm to 1 m is required.
The graphs of FIGS. 3a-3c illustrate them limits of conventional demodulation devices. The reference modulating signal Vmod is shown in FIG. 3a with its DC level, DC. The demodulation signal Vdemod is shown in FIG. 3b. It can be seen that certain modulations may not be detected, and that the modulations identified by the device are not identified perfectly, that is, the start and end of the modulation are not precisely located.
FIG. 3c illustrates, for a contactless device, a demodulator capable of precisely locating the start and the end of all the modulated signals emitted by the reader. To remedy these disadvantages, the Applicant has described in U.S. patent application Ser. No. 3,606 filed Mar. 25, 2000, a demodulator that is termed active based on the principle of detecting peaks and valleys. This active demodulator is able to generate, by following the modulated signal in a dynamic fashion, a low threshold and a high threshold which enables the start and end of the modulation to be precisely located.
Such a modulator is principally formed by two independent demodulators, F.E. and R.E. (FIG. 4), respectively optimized for the detection of the start and the end of the modulated signal. This demodulator comprises a peak detection cell DCR for extracting the reference modulating signal Vpeak1 from the modulated signal Vdb, a first demodulator FE for detecting the peak of the reference modulating signal Vpeak to generate a high comparison threshold and to locate the start of the modulation, and a second demodulator RE for detecting a valley of the reference modulating signal Vpeak to generate a low comparison threshold and to locate the end of the modulation. The demodulator further comprises a logic processing unit for providing the demodulated signal Vdemod.
For this purpose, peak detection is performed starting from a base cell diagrammed in FIG. 5. Such a cell essentially includes a diode D, a capacitor C and a DC current source I, and permits the reference modulating signal Vpeak to be extracted from the modulated signal Vdb.
Considering a full wave modulated signal Vdb, the capacitor C can only charge during the positive half cycle of Vdb, that is, when Vdbxe2x88x92Vpeak greater than Vd, wherein Vd is the threshold voltage of the diode. As soon as this relationship no longer holds, the diode of the base cell turns off and the capacitor C has a memory effect, that is, at the moment the diode turns off, Vpeak=Vdb(peak)xe2x88x92Vd very precisely. The capacitor C is then discharged at a constant current and Vpeak decreases according to the equation dVpeak/dt=i/C.
Consequently, it is necessary to choose a time constant that is suitable for recovering the modulated signal without attenuation, while limiting the resultant due to the carrier wave. The frequency of the modulated signal is slow with respect to that of the frequency.
The demodulator for peak and valley detection that has just been described has an input dynamic range that is limited by that of the comparator, which is between [Vtnxe2x88x92Vdd]. The useful level of the signal Vdb may reach voltages of 12 volts, while the comparator supply voltage Vdd is generally regulated between 3 and 5 volts. As a result, communication gaps appear at an elevated coupling with the reader.
In view of the foregoing background, an object of the present invention is to provide a demodulator for amplitude modulated alternating signals that do not exhibit communication gaps.
This object is achieved by using peak detectors for a comparator to detect the start and end of the modulation and by supplying the peak detectors and the comparator with an unregulated supply voltage.
The invention therefore relates to a demodulator for an amplitude modulated alternating signal, characterized in that it comprises a peak detection circuit for extracting the reference modulating signal from the modulated signal, and a first translations circuit for offsetting the level of the reference modulating signal by a value equal to the DC component to obtain an offset reference modulated signal. A circuit generates a comparison threshold to locate the start and the end of the modulation.
The demodulator preferably further comprises a comparator circuit for comparing the offset reference modulated signal with the comparison threshold, and for providing signals crossing the comparison threshold. An unregulated supply circuit preferably provides a supply voltage to the different circuits. This demodulator may further comprise a second translation circuit for offsetting the level of the signals provided by the comparators to the value of the regulated supply voltage.